In a known fuel cell system employing phosphoric acid electrolyte, cooler plates interposed between groups of fuel cells have a simple serpentine cooler flow path and utilize water coolant. Liquid water enters the cooler plates and a two-phase, water/steam mixture exits the cooler plates. A small fraction of the heat removal is due to increasing the sensible heat of the water as it increases to its boiling temperature, and a major fraction of heat removal is due to the latent heat of evaporation of liquid water to steam. U.S. Pat. No. 3,969,145 describes such a coolant system.
In any phosphoric acid fuel cell, the useful life of the fuel cell is determined principally by the rate at which phosphoric acid evaporates into the reactant gases and is not condensed back to a liquid before exiting the fuel cells. Non-reactive acid condensation zones at the reactant gas exits of the fuel cells minimize acid loss due to evaporation and thereby maximize life of the fuel cell stack. Such condensation zones are taught in U.S. Pat. Nos. 4,345,008 and 4,414,291, and in PCT patent publication WO 00/36680. The condensation zones should be below 140° C. (280° F.) in order to assure sufficient condensation of electrolyte so that the fuel cell stack will perform for at least ten years, which in turn requires that the coolant inlet temperature must be less than 140° C. (280° F.) in prior systems.
A competing problem in a phosphoric acid fuel cell stack is that the reformate fuel provided by a fuel processing system which converts various hydrocarbon fuels to hydrogen, such as a steam reforming fuel processor, contains between 0.3% and 1.0% carbon monoxide (CO), which is a poison to the anode catalyst and impedes the oxidation of hydrogen at the anode. The extent of poisoning is a function of the concentration of CO and cell temperature. At the likely concentrations of CO referred to hereinbefore, the temperature within the electrochemically active portion of each cell must be kept above 150° C. (300° F.) in order to provide reliable fuel cell performance. Thus, the temperature suited for condensation is lower than the temperature required for CO tolerance.